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Useful notes on the morphological features of the chlorophylls!
The cell wall:
In Chlorophyceae, the cell wall usually consists of 2 or 3 layers. The innermost layer is of cellulose and encircles the protoplast.
The second layer consists of pectic compounds of pectose sometimes associated with cellulose.
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In the case of Oedogonium and Cladophora, the outermost layer of the cell wall is chitinous in nature.
Image Curtsey: upload.wikimedia.org/wikipedia/33/Clyndrospermum.JPG
In the cell wall of Siphonales and Conjugales, the outermost wall layer is covered over with mucilaginous substance and becomes slimy, e.g., Spirogyra. Usually calcium silicates and carbonates are deposited on the mucilaginous layer. Some of the more primitive Chlorophyceae lack cell walls and the protoplasts are naked.
The protoplast:
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In motile cell, two or more contractile vacuoles are found which pulsate alternately and help in excretion. Usually the filamentous forms possess a single large central vacuole in each cell. In some Chlorophyceae, e.g., Prasiola, the vacuoles are not found. The vacuole is filled up with cell sap rich in tannins. The cytoplasmic connections are clearly seen in colonial forms such as Volvox. Such plasmodesmata are not found in filamentous forms but in Trentepohlia.
The chloroplast:
In Chlorophyceae, a variety of chloroplasts is known. In the genus Chlamydomonas, various types of chloroplasts are found. The detailed account of the chloroplasts of Chlamydomonas will be given in the description of the genus. The typical ‘chlamydomonad type’ of chloroplast is basin-shaped.
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Fig. 4.1 Ultra structures of the chloroplasts of Chlorophyceae (green algae). A discoia chloroplast showing grana-like arrangement of thylakoid cup-shaped chloroplast of Chlamydomonas.
Stellate chloroplasts are found in the cells of Zygnema and thallus of Prasiola. In Oedogoniales and Cladophorales, the chloroplasts are reticulate having many pyrenoids in them at anastamoses. In Spirogyra, the chloroplasts are ribbon-like and 1-14 in number in each cell. They are spiral and sometimes cut on the margins. In most of the Chlorophyceae, one chloroplast is found in each cell but exceptions are there. In Chlamydomonas alpina, several discoid chloroplasts are developed by breaking a single cup-shaped chloroplast.
In Mougeotia, special type of movement of chloroplast is found. Each chloroplast has independent capacity of its movement. The chloroplast rotates to a 90° and exposes its edge to the light.
The pyrenoids:
The pyrenoids are found in the chloroplasts of Chlorophyceae with few exceptions such as Chara, Voucheria, Microspora, etc.
There are different views about the functions of pyrenoids. According to one view, the pyrenoids are regarded as reserve proteins. According to other view, they are regarded as special organellae of the cells associated with metabolic activities. According to one rejected view, they are secreting receptacles.
The pyrenoids are crystalloid in structure and sometimes called ‘pyrenociystals’. According to some algologists, the crystals are embedded in a substance.
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The compound pyrenoids have been reported from the species of Tetraspora and Enteromorpha. The compound pyrenoid is surrounded by a starch sheath. There are as many starch grains deposited around the pyrenoid and the whole pyrenoid is surrounded by a starch sheath. Whenever, the starch sheath of starch grains is incomplete around the pyrenoid, the structure is called the ‘polar pyrenoid’.
In majority of Chlorophyceae, the pyrenoids are responsible for starch formation. In order Siphonales where the pyrenoids are lacking, the starch is formed by leucoplasts in the cytoplasm.
The pigments:
The most important pigment of Chlorophyceae is chlorophyll a. In addition to it chlorophyll b, carotenoids and xanthophylls are also present. Haematochrome is reported from Trentepohlia and because of this pigment this looks orange red in colour. Phycoporphyrin is recorded from several Conjugales. Fucoxanthin has been found in Zygnemapectinatum. The lutein is among principal xanthophylls.
The nucleus:
In majority of Chlorophyceae, the cells are uninucleate except Cladophorales where the condition is multinucleate. The multinucleate condition is also seen in Siphonales where the filaments are without septa. Fundamentally the nucleus of Chlorophyceae resembles with the nucleus of higher plants. The nucleolus is deeply stainable and conspicuous. It appears as a central caryosome. The centrosomes have been recorded from Volvocales and Cladophora. Mitosis and meiosis occur in all types.
The flagella:
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The flagella of Chlorophyceae and especially of order Volvocales are produced by an apparatus called ‘Neuromotor or flagellar apparatus’. Usually two flagella occur at the anterior end of the motile body. At the base of each flagellum, there is a flagellum producing, granule called ‘blepharoplast’.
These two blepharoplasts are connected to each other by a thin fibril called paradesmose. One of the blepharoplasts is connected by a long fibre called rhizopiast to the centrosome situated inside or outside the nucleus. The whole apparatus is called ‘flagellar or neuromotor apparatus’.
In Chlorophyceae, both the flagella are of whiplash or acronematic type. The terminal end of the flagellum is thin and uncovered called the end piece, whereas the basal part of the flagellum is comparatively longer and covered by a cytoplasmic coating. This part is called axoneme.
Under the electron microscope, the flagellum is seen to consist of a peripheral cylinder of 9 doublet fibrils surrounding a central pair of singlet fibrils. This type of arrangement is known as 9+2 arrangement. The entire group of fibrils is enclosed in a double sheath and the two central fibrils have subsidiary (single) sheath of their own.
The eye spot or stigma:
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Such spots are universally found on the motile organisms of the Chlorophyceae. They are lens-like and sensitive to light. The details of the stigma may be read from order Volvocales.
